As environmental regulations become progressively more stringent, new techniques and approaches are needed for dealing with difficult contaminants. For example, the required destruction and removal efficiencies (DREs) for some environmental pollutants, such as toluene diisocyanate (TDI), dioxin, dibenzofurans and polychlorinated biphenyls (PCBs) are extremely high. Conventional methods such as carbon adsorption or liquid scrubbing are not a complete remediation solution due to the fact that they simply transfer contaminants from one medium (i.e. water or air) to another (ie. solid carbon or scrubbing liquid). On the other hand, incineration and catalytic thermal oxidation present their own limitations. For example, the widespread production and use of chlorinated compounds in the industrially developed countries has resulted in large amounts of halogenated organic contaminants to seep into the soil, water and air. Incineration and even thermocatalytic oxidation of wastestreams containing halogenated compounds in many cases produce emission of products of incomplete combustion (PIC) such as dibenzofurans, dioxin and other pollutants that are known or suspected carcinogens. It is to be understood that in the terminology of this application "target species/compounds" denote those entities contained within the contaminated stream that are targeted for complete destruction and removal.
The past two decades has seen rapid growth and promulgation of new remediation technologies. In particular, a class of pollution control technologies known as the advanced oxidation processes (AOPs) has been the focus of much research and development. Among AOPs, those that employ ultraviolet (UV) radiation in conjunction with active oxidants (i.e. ozone, hydrogen peroxide, hydroxyl radical, superoxide ion radical, etc.) to accomplish mineralization of the target organic contaminants are of special interest. Generally, UV/AOPs are characterized with respect to the type of either the catalyst and chemical reactions involved (ie. homogeneous vs. heterogeneous) or light source employed (i.e. solar vs. artificial).
In general, UV/AOPs for treatment of the hazardous organic contaminants (HOCs) in fluids (both gas- and liquid-phase) comprise the following steps:
In the first step, an organic contaminant (hereafter called "primary reactant" or "target compound") that is adsorbed on the catalyst surface or resides within the fluid reacts to form products (hereafter termed "intermediate" or "secondary" products).
In the next step, the secondary products react to form other products (hereafter called "tertiary products" or "final products") that can be regarded as more benign, safer, or less detrimental to health and environment. The tertiary products are formed through a sequence or stepwise reaction scheme and an effective way to obtain tertiary or final products is to use specially engineered catalytic reactors disclosed in this document.